1. Technical Field
The present invention relates to an exposure device and an image forming device.
2. Related Art
An exposure device of the laser ROS (Raster Output Scanner) method, that scans by a polygon mirror light that is emitted from a laser light source, is conventionally used as an exposure device that writes a latent image onto a photoreceptor drum in copiers, printers and the like that form images by the electrophotographic method. Recently, exposure devices of the LED method that utilize light-emitting diodes (LEDs) as the light source are mainly being used instead of exposure devices of the laser ROS method. An exposure device of the LED method is called an LED print head, and is abbreviated as LPH.
An LED print head has an LED array in which numerous LEDs are arrayed on an elongated substrate, and a lens array in which numerous refractive index distribution type rod lenses are arrayed. Note that, here, “array” means a row of elements in which elements such as plural LEDs or plural lenses or the like are arrayed in a one-dimensional form or a two-dimensional form. In an LED array, numerous LEDs are arrayed in correspondence with the number of pixels in the fast scanning direction, for example, 1200 pixels per inch (i.e., 1200 dpi) are arrayed. A cylindrical rod lens exemplified by a SELFOC™ is used as the refractive index distribution type rod lens.
At the LED print head, the lights emitted from the respective LEDs are collected by the rod lenses, and an erect equal magnification image is imaged on a photoreceptor drum. Accordingly, a scanning optical system of the laser ROS method is not needed, and the structure can be made much more compact than a structure in accordance with the laser ROS method. Further, a driving motor that rotates a polygon mirror also in unnecessary, and there is the advantage that mechanical noise does not arise.
Several techniques using a hologram element array instead of rod lenses in LED print heads have been proposed.
LED print heads using LED arrays are generally used as exposure devices of the electrophotographic method, and therefore, this type of exposure method is usually called the “LED method”. However, because there is no need to limit the light-emitting elements to LEDs, hereinafter, the “LED method” will, for convenience, instead be called the “light-emitting element array method”.